A photovoltaic panel converts radiation energy into electrical energy. Of particular interest is the large-scale and cost-effective conversion of solar radiation (sunlight) into electricity using arrays of photovoltaic cells assembled into photovoltaic panels.
Thin-film photovoltaic panels are typically manufactured via a multi-step process, one stage of which is the assembly of a photovoltaic laminate on a substrate. When a transparent substrate is used for the sun-facing side of a panel, it is desirable to reduce the reflectivity of the substrate surface in order to allow more sunlight to reach the photovoltaic cell and be converted to electricity.
Photovoltaic laminates, which comprise one or more photovoltaic junctions disposed between front and back electrodes, are largely opaque to light transmission, due to the high light-absorption of the semiconductor junction and the presence of highly reflective metallic back electrode layers.
During the manufacture of photovoltaic laminates, layers of the laminate are deposited on the substrate surface, often extending to the substrate edges. The laminate is conductive, and if not removed from the substrate edges, it can lead to electrical shorts with the panel frame. The edge region is also vulnerable to environmental corrosion. Therefore it is necessary to electrically isolate the edge region of the substrate from the interior of the laminate and remove the laminate from the panel edges. Electrical isolation of a photovoltaic panel is conventionally achieved by using a laser to cut isolation grooves of a few hundred microns width through the photovoltaic laminate around the panel edges. Edge-deletion from the substrate surface at the panel periphery (e.g., from the edge to up to 1.5 cm into the substrate) can be achieved by laser or mechanical means.
Normally, light impinging on the panel can only transmit through the panel at the narrow scribe breaks where the back electrode/junction stack is divided. As a result, less than 1% of the sunlight is transmitted through the photovoltaic panel. In some applications, it is desirable to customize the degree of panel transparency and/or the light transmission pattern. For example, a significant amount of light transmission (20-50%) may be required for window or sun-roof installations. It may also be desirable to customize the color or tone of the transmitted light to match or contrast with the interior or exterior surroundings of the partially transparent photovoltaic panel.
A semi-transparent photovoltaic panel has been described in which transparent conductive oxides are used for both the front and back electrodes of the laminate. The degree of transmission can be regulated by adjusting the semiconductor band gap and thickness.
It is also known to fabricate a collection of holes or other polygonal apertures on at least the metallic back electrode to facilitate passage of light through the photovoltaic laminate. The junction layers can also be removed at the apertures to enhance light transmission. The apertures can be fabricated by photolithography using a photoresist layer.
It is also known to fabricate a translucent photovoltaic sheet on flexible stainless steel or polymer substrates. When metallic or polymer substrates are used, light must impinge from the film side of the substrate through a transparent conductive oxide (TCO) electrode on the light-facing surface of the laminate, rather than through the substrate. Small round apertures passing through the semiconductor layers and the substrate let a portion of incident light pass through. Aperture formation can be achieved by wet etching, laser drilling or mechanical perforation.
Partially transparent photovoltaic panels equipped with parallel slots cut into the opaque back electrode or electrode/junction stack have also been disclosed. A lift-off method, photolithographically defined etching, or laser drilling can be used to create the groove-shaped apertures.
Reflectivity of the sun-facing substrate surface is conventionally reduced by use of a multi-layer dielectric coating, chemical etching, or sol-gel coating methods. Typically, an anti-reflective coating is created or deposited on the substrate surface prior to the deposition of the photovoltaic laminate.
There remains a need for a method to produce partially transparent thin-film photovoltaic panels that is easy to use, cost-effective, efficient and adaptable to the specific application of the photovoltaic panels. There is also a need to streamline the separate processes required to electrically isolate, edge-delete, and reduce the reflectivity of the sun-facing side of the photovoltaic panel.